The Effects of Membrane Physical Properties on Insulin Receptor Autophosphorylation and Signalling

Abstract

<p>Many membrane functions are modulated by the bulk biophysical properties of the membrane. Various compounds which alter membrane physical properties were investigated for their ability to modulate insulin receptor autophosphorylation and signaling. Compounds of diverse structure, which raise the lamellar to inverted hexagonal phase transition temperature (Tʜ) in model membranes, including carbobenzoxy-dipeptides, apolipoprotein A-I, acyl carnitines, and lysophosphatidylcholine, inhibited insulin-stimulated tyrosine phosphorylation of isolated receptors, as well as of receptors in cells over-expressing human insulin receptor. For compounds of similar structure, the inhibition of insulin receptor tyrosine phosphorylation correlates well with their bilayer stabilizing potency. All of the compounds tested exert their effects independently of changes in insulin binding to the receptor or changes in the basal tyrosine kinase activity of the receptor. We suggest that the membrane additives tested affect insulin receptor activity through alterations of the bulk physical properties of the membrane. This is further supported by the inability of these compounds to influence the soluble insulin receptor kinase domain activity, measured in the absence of phospholipids. In addition, stereoisomers of dipeptide analogues had identical effects on insulin receptor phosphorylation, suggesting that their mode of action did not involve specific interactions with the receptor.</p> <p>Most of the compounds which inhibit tyrosine phosphorylation of the insulin receptor also inhibited glucose uptake in the same cells. Insulin-stimulated fluid phase pinocytosis was also inhibited by peptides which raise Tʜ. This suggests that alteration of membrane physical properties affects the divergent pathways involved in insulin receptor signal transduction.</p> <p>Compounds which lower the bilayer to hexagonal phase transition temperature in model membranes enhanced insulin stimulation of autophosphorylation in isolated receptors, with no effect on insulin receptor phosphorylation or signalling to glucose uptake in intact cells. The effects of cationic amphiphiles were not readily predictable from their membrane modulating activity.</p> <p>This thesis provides evidence that the mechanism of modulation of insulin signalling by these additives lies in the ability of such compounds to alter the bulk physical properties of the membrane. The results suggest that membrane monolayer curvature strain is a factor contributing to the efficiency of insulin signal transduction.</p>